Installing step flashing - Fine Homebuilding Article
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Installing step flashing

The first rule of roof flashing is water runs downhill; the second rule is that the first rule isn't without exceptions

Basic materials get the job done: Roofing caulk, galvanized roofing nails, tin snips, and step flashing (either flat or prebent). Basic materials get the job done: Roofing caulk, galvanized roofing nails, tin snips, and step flashing (either flat or prebent).

Installing step flashing correctly is one of the first things I teach a new member of my crew. The first rule of flashing is that water runs downhill. The second rule of flashing is that sometimes rule No. 1 isn’t the whole truth and that water also runs uphill, as when it wicks up inside debris or backs up behind ice. For this reason, I like to lap all flashing by at least 2 in.

It’s important to remember that a building always is moving. If you nail step flashing to the wall and to the roof, you’re asking for trouble. Most of the time, I like to avoid putting any extra holes in the roof surface, so I nail step flashing to the sidewall only, where both the next piece of flashing and the siding will cover the nail head. The bottom corner, where you start the course of flashing, is made from two pieces of bent step flashing lapped over each other and caulked in place.

Corner flashing comes first

Step 1: Make corner flashing from step flashing. Once you’ve shingled up to the sidewall, cut a piece of step flashing at a 45º angle from the outside corner to the bent seam. Bend it down and back to sit flat on the corner. Then sink two nails in the wall near the top, one on each face.

Step 2: Lay a bead of sealant. Where the next piece of flashing will overlap, apply a bead of caulk to seal the corner. This spot is prone to leaking because there is not a full 2 in. of overlap. Use a sealant designed for roofing. Not all caulks can withstand the heat and exposure of being on the roof.

More Info

Instead of making your own bottom-corner flashing from two pieces that must be cut and caulked together, you can flash the bottom corner with a piece of corrugated aluminum made by amerimax home products ( The flashing has a 31/2-in. base (which rests on the roof) and a 2-in. stand (which fits against the wall). it can be twisted or bent to fit any roof pitch and can be used for left-hand or right-hand applications. generally available at larger home stores, a single piece sells for less than $3.

The flashing alternates with the shingles

Step 3: Bend the first piece along the plumb line. The first piece of step flashing needs one bend so that it laps cleanly over the corner flashing installed in the previous step. Make sure the caulk joint between these two pieces is bedded evenly. Then sink one nail into the sidewall to hold the step flashing in place.

Step 4: Begin the weave. With step flashing, you do a little flashing, then a lot of roofing, then a little more flashing, and so on. Each piece of step flashing laps over the shingle below and under the shingle above. The bottom edge of the flashing should extend just below the nail line. Attach each piece with a single nail high enough to be covered by the next course of flashing, the building wrap, and the siding.

The top requires another custom piece

Step 5: This roof ends in a peak. For the first side, cut the step flashing along the fold line, and bend down the lower flap. Drive one nail to hold it in place.

Step 6: Caulk the top. Once you’ve come up the other side with step flashing and shingles, you’re ready to put on the final piece. Apply a vertical bead of caulk as shown.

Step 7: Make the last piece of flashing. This is the brother to the one you already put on the peak. Cut along the fold line, bend down the flap, and press the pieces together along the caulk joint. Sink one nail into the wall to secure the last piece.

Step 8: Once you’re done, half of the flashing will be covered by shingles. After the building paper and the siding go on, almost all of it will be covered, which is one of the main reasons flashing has to be installed carefully to begin with.

Photos by: Krysta Doerfler; drawings by: Clark Barre
From Fine Homebuilding182 , pp. 120-124
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